As a typical conventional technique concerning an optical access network may be cited PON (Passive Optical Network) defined by IEEE 802. 3ah “Draft Amendment to Carrier Sense Multiple Access with Collision Detection (CSMA/CD) access method and physical layer specifications”, Oct. 7, 2003: a standard of IEEE (Institute of Electrical and Electronics Engineers). The PON transmits a frame defined by IEEE802. 3ah (hereinafter referred to as an IEEE 802. 3ah frame) or an Ethernet (registered trademark) frame at a gigabit rate. The conventional technique will be herein referred to as a GE-PON.
According to IEEE 802. 3ah, the GE-PON comprises an OLT (Optical Line Terminal) as a center terminal, ONUs (Optical Network Units) as remote terminals, an optical splitter, and optical fibers for connecting them. Generally, the OLT is placed in an office of a carrier, while the ONU is placed in user's home.
FIG. 1 is a diagram schematically showing an example of the construction of the GE-PON. In FIG. 1, an n1×1 optical splitter is employed as the optical splitter. Incidentally, n1×1 indicates that the splitter is provided with one input (output) port and n1 (n1: an integer; n1=1, 2, 3, . . . , n) output (input) port(s). Each port serves as an input port or an output port according the destination of a signal, i.e. according to whether a signal is transmitted to the 1 side or the n1 side. Accordingly, two sides of the optical splitter or the optical switch will not be herein referred to as input/output sides, but referred to as 1 side and n1 side. In addition, a signal transmitted from the OLT to the ONU will be referred to as a downlink signal, while a signal transmitted from the ONU to the OLT will be referred to as an uplink signal.
The OLT is connected via a single optical fiber to the 1 side of the optical splitter. The ONUs are each connected via a single optical fiber to the n1 side of the optical splitter. Incidentally, the optical splitter used herein is a passive device, and therefore, such a system is generally called PON (Passive Optical Network).
In the GE-PON, the direction from the OLT to the ONU is defined as the downlink direction, and light with a wavelength of 1.5 μm is utilized. On the other hand, the direction from the ONU to the OLT is defined as the uplink direction, and light with a wavelength of 1.3 μm is utilized. With a single optical fiber, both-way or two-way communication is performed according to the WDM (Wavelength Division Multiplexing) system. The section between the OLT and the optical splitter is referred to as Feeder section, while the section between the optical splitter and each ONU is referred to as Drop section.
FIG. 2 is a diagram showing the construction of an IEEE 802. 3ah frame. The IEEE 802. 3ah frame as shown in FIG. 2 is used for a signal (packet) communicated between the OLT and the ONU. In the direction from the OLT to the ONU (downlink direction), an IEEE 802. 3ah frame transmitted from the OLT is split as an optical signal by the optical splitter, and arrives at all the ONUs connected to the optical splitter.
Each ONU checks the LLID (Logical Link Identifier) of the IEEE 802. 3ah frame shown in FIG. 9 to determine whether or not the frame is directed thereto. When the ONU has determined that the IEEE 802. 3ah frame is directed thereto, it accepts the frame. On the other hand, when the ONU has determined that the IEEE 802. 3ah frame is not directed thereto, it discards the frame.
In the direction from the ONU to the OLT (uplink direction), the OLT performs transmission control for the ONUs connected to the optical splitter so that IEEE 802. 3ah frames transmitted from the respective ONUs do not collide with one another on the optical fiber between the OLT and the optical splitter or in the Feeder section.
More specifically, the OLT instructs each ONU the time at which the IEEE 802. 3ah frame is to be transmitted. The ONU has to transmit the IEEE 802. 3ah frame according to the instruction. Since the OLT performs the transmission control for the respective ONUs, IEEE 802. 3ah frames therefrom do not collide with one another in the Feeder section. Thus, the IEEE 802. 3ah frames can be multiplexed on the time axis and arrive at the OLT.
As is described above, the PON allows a plurality of ONUs to share a network in the Feeder section on the OLT side. In other words, a plurality of ONUs connected to the optical splitter can share one optical fiber and the OLT. Thus, the PON attracts attention in view of the economization of an optical access network.
The GE-PON, however, has three problems as follows.
First, the optical splitter applied to the PON generally divides optical power, which loses signal strength considerably and so provides less transmission distance between the OLT and the ONU. This tendency is especially remarkable when “n” of the n×1 optical splitter is a large value. This loss is the structural one in the optical splitter and an essential characteristic of the device. Therefore, the loss cannot be avoided. In other words, since the optical splitter divides input optical power, in the case of a 2×1 splitter, the optical power is halved, resulting in at least a loss of 3 db. In the case of a 4×1 splitter implemented by two stages of 2×1 splitters, at least a loss of 6 db occurs. In addition, considering the manufacturing loss and the like, the loss is around 8 dB. Similarly, in the case of an 8×1 splitter implemented by three stages of 2×1 splitters, at least a loss of 9 db occurs. In addition, considering the manufacturing loss and the like, the loss is around 11 dB. Further, in the case of a 16×1 splitter implemented by four stages of 2×1 splitters, at least a loss of 12 db occurs. In addition, considering the manufacturing loss and the like, the loss is around 14 dB. Still further, in the case of a 32×1 splitter implemented by five stages of 2×1 splitters, at least a loss of 15 db occurs. In addition, considering the manufacturing loss and the like, the loss is around 17 dB.
The optical splitter loss substantially limits the transmission distance between the OLT and the ONU. With PX-10 defined by IEEE 802. ah as an optical condition, the transmission distance between the OLT and the ONU will be estimated. According to the PX-10 optical condition, a difference in optical power between transmission and reception in the downlink direction is 21 dB. Besides, it is assumed that the sum of connector loss, fusion splice loss, etc. in an optical fiber between the OLT and the ONU is 3 dB, the loss in 1 km of an optical fiber is 0.3 dB and the distribution loss in user's home is 1 dB. Incidentally, the values assumed here are appropriate in a practical optical access network.
In this case, if a 32×1 optical splitter is utilized, (allowable optical fiber loss Lf)=(PX-10 transmission-reception power difference)−(32×1 optical splitter loss)−(the sum of connector loss, fusion splice loss, etc. in an optical fiber)−(distribution loss in user's home)=21−17−3−1=0 dB. Then, the transmission distance D between the OLT and the ONU is 0 km: D=0 km. Besides, if a 16×1 optical splitter is utilized, allowable optical fiber loss Lf=21−14−3−1=3 dB, and the transmission distance D between the OLT and the ONU is 10 km: D=3/0.3=10 km.
As is described above, the optical splitter loss limits the transmission distance between the OLT and the ONU. Additionally, n×1 optical splitter loss increases as the value of “n” becomes larger. An efficient way to operate the PON economically is to increase the value of “n” as possible. However, although PX-10 as an optical condition is applicable to the case where n=16, it cannot be applied to the case where n=32.
Second, the GE-PON has a security problem. As described previously, a downlink signal (IEEE 802. 3ah frame) from the OLT is split by the optical splitter, and arrives at all the ONUs connected to the optical splitter. Therefore, a user who has the ONU can view a signal to another user's ONU intentionally or with malice. In the PON, a signal transmitted from the OLT to the ONU is generally encrypted to prevent this. Also, encryption is currently being standardized for the GE-PON. However, even if a signal is encrypted, it would be decrypt, and the security problem cannot be solved.
Third, one ONU can interfere with the communication of another ONU. As described previously, the OLT instructs each ONU the time at which an IEEE 802. 3ah frame is to be transmitted, and the ONU transmits the frame at the time so that IEEE 802. 3ah frames from the respective ONUs do not collide with one another on the optical fiber between the OLT and the optical splitter or in the Feeder section. That is, the ONU transmits an IEEE 802. 3ah frame only at the specified time, and not at the time, terminates the transmission of an optical signal. However, if a user who has the ONU does not follow the instruction intentionally or with malice and continues to transmit optical signals from the ONU, the optical signals are always present in the Feeder section. Consequently, the communication of other ONUs is totally disabled.
Further, even if the user does not continue to transmit optical signals from the ONU intentionally or with malice, when failures occur in the optical transmitter of the ONU and the transmitter continuously transmits optical signals, the same problem arises.
There have been proposed techniques concerning the aforementioned problems in the conventional GE-PON. Japanese Patent Application laid open No. HEI9-153858 discloses a technique to solve the problem that “to avoid the collision of uplink optical signals from a network slave station to a network master station that occurs frequently at an optical star coupler, the arrival of information at the network master station delays”.
According to the technique, a plurality of network slave stations are connected to a network master station via star-shaped optical fibers. When one of the network slave stations transmits an optical signal to the network master station, the transmission is detected to connect the optical fiber of the slave station to the master station.
That is, the network of the master and slave stations is configured with a 1×n optical star coupler and a multiposition optical switch. A decoding circuit (address determination circuit) specifies the originating network slave station to energize the switch coil of the multiposition optical switch. Thereby, the network master station is connected to the network slave station and disconnected from the other slave stations. Thus, the collision of uplink optical signals is prevented.
The network may be configured with an optical rotary switch in place of the optical star coupler and the multiposition optical switch. In this case, the decoding circuit discriminates the identification signals of the originating network slave stations fed from a driver circuit. According to the output of the decoding circuit, the optical rotary switch is switched to implement the connection of only necessary circuits.
However, the technique employs the optical star coupler, and therefore, the problem as to the transmission distance is still unsolved. Besides, with the construction using the optical rotary switch, it is possible to avoid the collision of uplink optical signals in the Feeder section. The optical rotary switch, however, neither selects the destination of a downlink optical signal nor connects the signal to the destination.
In addition, Japanese Patent Application laid open No. HEI10-70509 discloses a technique to solve the problem in the three types of conventional optical subscriber line systems: SS (Single Star) system, ADS (Active Double Star) system and PDS (Passive Double Star) system. The SS system has a problem of high cost, the ADS system has a problem of a limitation on provided services, and the PDS system has a security problem.
According to the technique, to solve the problems mentioned above, a time slot controller assigns a time slot to the line terminating equipment CT of an exchange for each subscriber based on the ADS system. The CT and network terminating equipment ONU in each subscriber's home are provided with multi-rate burst converters for inserting/extracting fast or slow information to/from this time slot. Further, a remote terminal RT located between the CT and ONU is provided with a space-division optical switch for switching the time slot as an optical signal.
That is, the optical subscriber line system is configured with a space-division optical switch, and has a time slot structure. With the time slot structure, the space-division optical switch is controlled to ensure the connection to only necessary subscribers.
This technique solves the problems of security and communication interference. However, the time slot structure complicates the system construction. Additionally, since it is not clear whether or not subscribers can be increased by multistage space-division optical switches, the problem as to the transmission distance is probably unsolved.